Republican presidential candidates battled to win over undecided conservatives as Iowa's first vote of the nominating process to challenge President Barack Obama looms next week.

Only presumed-front runner and former Massachusetts Governor Mitt Romney seemed to largely escape attack as he worked to win a state that long seemed out of reach until now.

Five days out, public and private polling show Mr Romney and Texas Representative Ron Paul in strong contention to win the Iowa caucuses.

They are the only two with the money and the organisations necessary to ensure big turnouts on Tuesday.

Mr Obama is vulnerable as he seeks a second term, weighed down with voter dissatisfaction over his handling of the economy and the stagnant recovery from the Great Recession of 2007-2009.

Mr Romney now is making an unabashed push in Iowa. His rivals are scrambling to deny him momentum heading into the January 10 primary in New Hampshire, his second home.

Mr Paul, the 76-year-old libertarian-leaning Texan, drew about 500 people at the Iowa State fairgrounds late on Wednesday. A group of Occupy Wall Street activists tried to interrupt the rally, but that was not the main surprise.

State Senator Kent Sorenson, who had campaigned a few hours earlier with Michele Bachmann as a state chairman of her bid, announced he would support Mr Paul instead.

Mr Paul's anti-government appeal appears to tap into the desire of a frustrated electorate for profound change in an era of high unemployment and weak economic growth.

Three others - former Pennsylvania senator Rick Santorum, Texas Governor Rick Perry and former speaker of the House of Representatives Newt Gingrich - were also trying to convince fickle conservatives who may feel Mr Romney is too liberal and Mr Paul is too far out of the mainstream.